Evaluation of disk susceptibility testing of cefotaxime/desacetylcefotaxime

The susceptibility of 453 clinical bacterial isolates was determined by broth microdilution and disk diffusion methods to cefotaxime (CTX), desacetylcefotaxime (dCTX), and various combinations of CTX plus dCTX. With a 1:1 ratio of CTX/dCTX, 97% of the minimal inhibitory concentrations were within one doubling dilution of those for CTX alone. With the disk diffusion test, the 30/30 micrograms disk of CTX/dCTX produced zone diameters very similar to those of the 30 micrograms CTX disk, averaging 0.4 mm larger than the latter. Regression analysis demonstrated that the 30 micrograms CTX disk was equally predictive for CTX and a 1:1 ratio of CTX/dCTX.